The fundus of the eye, or retina, is a complex layered structure arranged in an approximately spherical shape at the back of the eyeball. It contains the light sensing rods and cones that enable vision. It is nourished by oxygenated blood supplied through arterioles and removed through venules. The nerve impulses from the rods and cones are directed to the brain through the optic nerve on the fundus, which corresponds to the blind spot.
Direct visual observation of the retinal fundus can be accomplished using an ophthalmoscope, an instrument that has been around in various forms for over 150 years. The ophthalmoscope employs a light source, means for coupling the light into the eye through the pupil, and means for collecting light reflected back from the fundus and presenting an image of the fundus to the observer. The eye responds to continuous light by constricting the pupil size and so reducing the amount of light available to form the image of the fundus. For this reason, the eye pupil may have to be chemically dilated, using a mydriatic, in order to facilitate imaging of the fundus.
A fundus camera is similar to the ophthalmoscope but provides a permanent record of the fundus image in the form of a photograph. It also enables the use of a short, powerful flash of light to replace the continuous light required for the ophthalmoscope, and so sometimes avoiding the need for a mydriatic. The fundus camera uses an electronic image sensor such as a charge-coupled device (CCD) and the image can be stored electronically. The image may also be displayed on a monitor or printed out as a photograph.
The fundus image is dominated by the appearance of the optic nerve and the vascular structure of arterioles and venules. It is substantially of the colour red, this coming from the blood, with some regions having an orange or yellow bias. The ophthalmologist is able to use the fundus image to aid in the diagnosis of the health of the eye. Thorough diagnosis requires the use of a battery of other oculometric instruments in addition to the fundus camera.
The fundamental limitations of fundus imaging as a diagnostic tool are rooted in the subjective nature of the image evaluation and in the substantial variations in the image that result from the uncertainties of many of the parameters that are integral to the imaging process and presentation.
The colour perception of the human eye is variable. No two people perceive the same colour image in the same way, and in some cases, one may suffer from a form of colour-blindness, commonly an inability to distinguish red from green. As there is only a very minor blue component in a retinal image, red-green colour blindness effectively removes all colour information, and a technician having such colour blindness cannot properly assess a retinal image. The colour perception of the human eye is also conditioned by the intensity and spectrum of the environmental lighting; the background illumination may come from daylight, some form of fluorescent lighting, or incandescent lighting.
Similarly, the colour presentation of images using photographs or electronic displays is variable. Any photograph or display is limited by the gamut of colours enclosed by the specific three primary colours employed. The process and manufacturing tolerances will result in a spread from one photograph or display to another, which will be compounded by aging effects and the impact of environmental influences such as temperature.
Visual observation of the fundus is essentially a rudimentary form of multispectral imaging where the three colour channels correspond to those of the observing eye. The spectral sampling locations and widths of the three visual colour channels do not necessarily correspond with those that would be chosen in an optimal fashion determined by the reflection characteristics of the retina associated with specific retinal diseases or defects.
Potentially important information contained in small variations of the intensity or brightness of the image may be lost where the dynamic range of the display is limited. Such variations may be hidden in a white-out region or a darkened region of the retinal image, or simply missed as the human eye is limited in its ability to discern minor changes in intensity or brightness across the image.
The limitations of the display and the perception thereof are further compounded by the uncertainties associated with generating the image. The illumination source intensity and optical spectrum can vary from camera to camera, from time to time, and with the age of the instrumentation employed. This will result in concomitant variations in apparent image brightness. The sensitivity of the image sensor, be it film or electronic (e.g., a CCD), can also vary from unit to unit. This will also result in concomitant variations in apparent image brightness. The optical transmission efficiency of the eye is not always high, especially in the presence of cataracts. The transmission efficiency will also vary across the optical spectrum. This will result in concomitant variations in apparent image brightness and colour. The amount of illumination that is reflected from the retina and that returns to the imaging apparatus is strongly dependent on the size of the pupil. As the size of the pupil varies greatly from person to person and with environmental lighting conditions, this will further result in concomitant variations in apparent image brightness.
Further, the reflectivity of the retina can be strongly dependent on the ethnicity of the person, as a consequence of the different concentrations of melanin. People of African ethnicity have higher melanin concentrations resulting in low retinal reflectivity, which can lead to dark retinal images that are difficult to interpret.
Furthermore, during retinal fundus imaging, a patient is typically required to fixate on a target as one or more images of the retina are obtained. As the eye can move between images, no two images are likely identical and common structure between any two images can be several hundreds of pixels apart.
In addition, the retina is not an ideal spherical surface, and can be slightly deformed during a cardiac event (the pressure wave from a heart beat may induce a mechanical or reflective change and hence affect the perceived vessel position). This factor is of greater importance when the inter-study (examinations over time to establish longitudinal trends) analysis is performed
Ophthalmologists need to carefully track the progression of the retinal health problems of their patients in order to prescribe the most appropriate course of treatment. For this purpose, they carry out examinations over time to establish longitudinal trends. However, because of the variations and uncertainties listed above, the utility of fundus cameras for longitudinal monitoring is severely limited.
It is, therefore, desirable to provide a fast and efficient method and apparatus for registration of multispectral retinal images.